One use of the technology disclosed herein is to realize the ability by which one can remotely detect the presence of an optical beam which may contain temporal information, such as phase modulation, amplitude modulation, and vibrations of an object (the latter using non-contacting techniques involving a laser beam and an optical detection apparatus). One advantage of using the technology disclosed herein is that one can, for example, use it to measure very small phase shifts, including vibrations, on the order of fractions of an optical wavelength in amplitude, over a frequency range on the order of Hz to MHz or more. This minute displacement detection sensitivity corresponds to a phase-modulated optical beam, whose depth-of-modulation is on the order of 0.001 or less. Another advantage is that this system enables high-performance remote sensing and optical communications to be realized using eye-safe wavelength beams (1.5 μm). Yet another advantage of using this technology is that it enables one to detect vibrations from non-specular objects (e.g., surfaces with roughness features on the order of optical wavelengths) that reflect an incident optical beam into a diffuse set of angles, resulting in a highly speckled pattern of small spots. Still yet another advantage of using the technology disclosed herein is that the receiver possesses a set of high-gain, low-noise optical amplifiers as a front-end means to amplify the diffusely scattered beam that enters the receiver, thereby improving the shot-noise-limited sensitivity by two orders of magnitude relative to a conventional multi-speckle compensated vibrometer.
Yet another advantage of using the technology disclosed herein is that a system can be employed as a coherent receiver that can compensate for both static and dynamic propagation distortions imposed upon an incident optical signal, including, as an example, optical aberrations resulting from atmospheric turbulence, poor-optical quality elements, and differential phase shifts due to mechanical vibrations and/or thermal perturbations imposed onto the fiber array receiver itself.
The technology disclosed herein has potential use for remote sensing of vibrating objects without the need for physically contacting the surface under test. Thus a vibrometer can be used for remote sensing of threats (e.g., an Unmanned Aerial Vehicle (UAV) can interrogate objects down on the ground such as tanks, minefields, etc.; or, a ground-based platform, say, with a Directed Energy Weapon (DEW), can interrogate incoming threats to assess its functionality and operational state). In addition, this vibrometer can be used to interrogate objects on the ground (from an airborne location) to ascertain the nature of the materials that comprise the object in question, and, hence classify the object in terms of its functionality (in this case, a second laser, in conjunction with the device disclosed herein, can be employed to excite photo-acoustic modes in the material under interrogation). In addition, one can use the technology disclosed herein for real-time, manufacturing in situ process control, as well as in-service inspection of materials (structures, welds, bonds, etc.) and life-cycle evaluation of smart materials, the latter case referred to as health monitoring of infrastructures via “inspection on demand”.
The prior art includes single-speckle vibrometers as well as multi-speckle, compensated vibrometers. In the former case, a laser beam impinges on the object under test, and the receiver is designed to receive a single spatial mode, or speckle, which is then directed into a coherent detector (either a homodyne or heterodyne system). This system can be shot-noise limited in sensitivity. Nonetheless, its overall performance is reduced by ≈30 dB over multi-speckle receivers, since it is only capable of processing a single speckle, or spatial mode. The technology disclosed herein enables one to process many speckles, thereby enhancing the performance of the system. In the latter case, there exist a variety of multi-speckle vibrometer devices, such as self-referencing receivers (using a Fabry-Perot resonator as a multi-speckle FM discriminator), as well as multi-speckle vibrometers with adaptive optical front-end devices (such as 2-wave mixers, Spatial Light Modulators (SLMs), photo-emf sensors, etc.). In these cases, multiple speckles can be processed, but, at the expense in terms of sensitivity (the Fabry-Perot resonator), noise and throughput (in the case of 2-wave mixers), and sensitivity in excess of the shot-noise limit (the photo-emf devices). The technology disclosed herein enables one to realize shot-noise limited sensitivity, with improved performance beyond existing systems (owing to the front-end low-noise amplifiers), and with wavefront-compensation capability. The enhancements derive from the fact that the low-noise front-end fiber amplifiers also provide for adaptive optical compensation of wavefront distortions.
One aspect of the prior art involves single-speckle vibrometers. Another aspect involves multi-speckle vibrometers, but for these devices the front-ends are either totally passive in nature (such as the self-referencing Fabry-Perot resonators) or, at best, active in the sense that a collection of multiple speckles are processed via beam cleanup or real-time holography. However, it is believed that in none of these multi-speckle-based vibrometers is the notion of front-end amplification discussed or implied, rather the prior art focuses on methods to deal with the highly diffuse and speckled incident beam, without further active processing in such devices. The technology disclosed herein goes beyond the prior art, in that it not only provides a way of processing a highly speckled incident beam, for example, but, also, at the same time, the technology disclosed herein adds low-noise gain for enhanced shot-noise limited sensitivity by approximately 20 dB (fiber amplifiers can provide small signal gains at ≈40 dB without parasitics).
Another aspect of using the technology disclosed herein is that one can realize an effective large input aperture to an optical receiver or telescope without the need for a possibly costly, massive, highly polished transparent lens or mirror of high optical quality at the input plane. In essence, an array of coherently combined optical fibers essentially replaces a single, large optical element. Moreover, optical fibers are lightweight (low mass and small moment of inertia), highly flexible and readily available as low-cost elements. Hence, the technology disclosed herein can be employed to replace costly, bulky massive optical elements that would otherwise require secure and vibration-free mounting in high-cost gimbal devices for beam steering. The technology disclosed herein, owing to low moments of inertia and flexibility, can be serviced by or utilize a compact, low-cost gimbal requiring a low torque driver for beam steering. Finally, given the use of a large array of fibers, the technology disclosed herein can perform in the face of fiber failures, resulting in a graceful degradation of the receiver over its in-service lifecycle. By complete contrast, a damaged or shattered optical lens would, most likely, result in a non-functional optical system.